1. Field of the Invention
The invention relates to a document feeding apparatus for feeding a document to a predetermined position, an image reading apparatus having the document feeding apparatus, and an image forming apparatus having the image reading apparatus.
2. Description of Related Art
In an image forming apparatus such as a digital copying machine, a printer or a FAX apparatus, there is known an auto document feeder (hereinafter referred to as the ADF) disposed on a document glass stand and designed to automatically feed a document to a reading position on the document glass stand to read the image information of the document. Because of the recent rapid digitization and due to the development of a high-speed digital copying machine by digital reading and memory mounting, and the mounting of the ADF onto a low-priced popular kind of machine, higher accuracy and higher speed image reading performance are required of the ADF as image input means for transferring an image to a personal computer or the like through a network.
As an ADF mounted on an image reading apparatus provided with two reading modes, i.e., an ADF mode in which the ADF is installed above a document glass stand and the flow reading of a document by the ADF is effected, and a book mode in which a document is set on the document glass stand and reading is effected with an optical system being moved, there is known one in which a document tray is disposed above a delivery tray and a document feed path leading from the document tray to the delivery tray past a reading position is constituted by a U-turn path to thereby decrease occupation space and achieve the downsizing of the apparatus.
An ADF of this type is provided with a transparent film member as a member for guiding the surface side of a document. As the advantages of this type, mention may be made of the following:
(1) Unlike a case where a member for guiding the outer side of a document is comprised of a plurality of members, a document can be transported smoothly from the upstream side of the reading position to the delivery tray via the reading position. Particularly, the shock when the leading edge of the document strikes against a delivery guide member for guiding the document from the reading position to the delivery tray can be mitigated.
(2) It is not necessary to provide a construction for scooping up the document from the reading position to the delivery guide member and therefore, there is not the necessity of providing an ADF reading position discretely from the image reading area of the book mode, and the further downsizing of the apparatus can be achieved by a simple construction.
FIG. 9 of the accompanying drawings shows the construction of an example of an ADF in which the document feed path is constituted by the U-turn path as described above and a transparent film member is used as a document surface side guide member. Also, FIG. 10 of the accompanying drawings is an enlarged cross-sectional view of the essential portions of the ADF, and FIG. 11 of the accompanying drawings is a cross-sectional view taken along line XI—XI in FIG. 10. It is to be understood that the ADF mentioned as an example is mounted on an image forming apparatus (not shown). This example of the conventional art will hereinafter be described with reference to FIGS. 9 to 11.
The reference numeral 1 designates a document tray, and the reference numeral 2 denotes a width regulation plate for regulating the side of a document. The reference numeral 3 designates a pick-up roller, which can be moved down to a position for contacting with the document (a position 3′ on the upper surface of the document in FIG. 10). The reference numeral 4 denotes a separation roller, which is always in contact with a separation pad 5 opposed thereto and effects frictional separation to thereby feed the documents one by one. The reference numeral 6 designates a first feed roller, and the reference numeral 7 denotes a feed runner.
A separated document is fed along a feed path formed by an inner guide 10 and an outer guide 18 and downwardly curved and inclined, and is transported to a reading position Q via an ante-reading roller 11 and a reading pressure runner 12 as upstream feed means immediately before the reading position Q. A guide member near the reading position Q for guiding the backside (the side opposite to a read side) of the document is referred to as a reading guide 13. A Mylar guide 14 as a transparent sheet, which is a transparent film member, is mounted as means located on a side opposed to this reading guide 13 and for guiding the front side of the document. The Mylar guide 14 is a transparent plastic film comprising, for example, a PET material or the like, and is formed with a thickness of the order of 100 μm. A member restraining the Mylar guide 14 by a boss portion is referred to as a Mylar holder 19. The Mylar guide 14 has one end thereof restrained by the boss portion 19a of the Mylar holder 19 and has the other end thereof resting on a delivery guide 20 via a document glass stand 62.
The reading guide 13 is such that the surface thereof opposed to the reading position Q is formed by a flat surface portion 13a parallel to the document glass stand 62, and an upstream inclined portion 13b and a downstream inclined portion 13c connected to the flat surface portion 13a on the upstream side and downstream side thereof and being curved and inclined. Also, the reading guide 13 is mounted for pivotal movement about a pivotal fulcrum 13d, and is biased toward the document glass stand 62 by biasing means (not shown). Also, as shown in FIG. 11, hit projections 13e as projected members are provided on the opposite ends of the flat surface portion 13a in a direction orthogonal to a feeding direction, and the minute interval “d” with respect to the Mylar guide 14 is kept constant. Accordingly, the Mylar guide 14 is in contact with the document glass stand 62 near the reading position Q, and the image of the document is read through the Mylar guide 14 and the document glass stand 62.
The document having passed the reading position Q is transported along the Mylar guide 14 mounted on an upwardly inclined delivery guide 20, and is delivered onto a delivery tray 17 by a delivery roller 15 and a delivery pressure runner 16 as downstream feed means. That is, in this ADF, a feed path leading from the document tray 1 to the delivery tray 17 past the reading position Q is formed by a U-turn path.
In the ADF wherein the feed path is formed by a U-turn path as described above, there has been the problem that the document is fluttered by a shock at the moment when the trailing edge of the document being transported has passed between a pair of rollers immediately before the reading position and as the result, the defect of image such as misregister or image distortion is liable to occur to the read image. The mechanism of this will now be described with reference to FIG. 12 of the accompanying drawings.
FIG. 12 illustrates the state at the moment when the trailing edge of the document passes an ante-reading roller. The document P is transported so as to be along the upstream inclined portion 13b and downstream inclined portion 13c of the reading guide 13 when the document P is transported while being nipped between the ante-reading roller 11 and the delivery roller 15.
This is because in order to prevent slack or the like of the document being transported, the peripheral speed of the delivery roller 15 is generally set at a level about 1% higher than the peripheral speed of the ante-reading roller 11. When the trailing edge of the document P has passed the ante-reading roller 11, the document P so far transported along the upstream inclined portion 13b of the reading guide 11 tends to be restored to its original state by the stiffness of the document and therefore falls toward the Mylar guide 14. By this shock, the document is fluttered up and down. The influence of this fluttering extends to the document at the reading position and as a result, the defect of image such as misregister or distortion occurs to the read image.
Against such problem, as described in Japanese Utility Model Application Laid-Open No. 7-42258 (see FIG. 13 of the accompanying drawings), there is a construction in which a presser plate 118 for directly pressing a document against an inner guide with a resilient force is provided upstream of the reading position to thereby suppress the vibration or fluttering of the document and achieve the stable transport of the document at the reading position.
In the construction wherein the document is directly pressed, however, the shock at the moment when the trailing edge of the document has passed the ante-reading roller 11 can be alleviated, but the shock at the moment when the trailing edge of the document passes the presser plate 118 newly causes misregister or distortion.